1. Field of the Invention
The present invention relates to a radiation detecting element. More particularly, the present invention relates to an element for detecting .alpha.-ray, .beta.-ray, .gamma.-ray, X-ray, neutron or UV light which is used in, for example, high energy physical experiments.
2. Description of the Related Art
Recently, as radiation detectors, semiconductor detectors comprising a semiconductor such as silicon are increasingly used in place of detectors which require a large space such as a gas drift chamber. With the semiconductor detector comprising silicon, a reverse voltage is applied to a p-n junction of silicon, and incidence of radiation generates electron-hole pairs, whereby, a current flows. The semiconductor detector has good linearity between the radiation energy and the detected current.
However, since silicon or germanium is used in a detecting part, saturation mobility of carriers is small, for example, saturation electron mobility in silicon is 1.times.10.sup.7 cm/sec., so that a detecting element has a low response speed. Highly pure silicon has a resistivity of in a order of 10.sup.5 .OMEGA..cm, and germanium has a lower resistivity. When a current flows through the element, since a dark current is very larger. Therefore, a p-n junction is formed and a voltage is applied in a reverse bias. However, in this way, a radiation is detected only in a depleted layer having a thickness of several ten .mu.m. Then, the element tends to have an insufficient detecting sensitivity in view of a collision cross section.
When a semiconductor in the detecting part of the radiation detecting element comprises single crystal diamond, the element has a high response speed and good sensitivity since the carriers have very large mobility in the single crystal diamond (e.g. 2.5.times.10.sup.7 cm/sec.) (see Japanese Patent Kokai Publication No. 198780/1987).
Recently, there is developed a method for synthesizing a film-form polycrystal diamond from a vapor phase. By using this method, it has been tried to produce a radiation detecting element comprising the polycrystal diamond, but no good result has been obtained, since the polycrystal diamond has very low sensitivity to the radiation.
The reason why the polycrystal diamond has very low sensitivity to radiation is that grain boundaries present between adjacent particles interfere movement of the electrical carriers generated by the radiation. Whenever the carrier passes the grain boundary, its speed is greatly decreased so that an effective mobility of the carrier is made small. Therefore, performances of the radiation detecting element comprising the polycrystal diamond are only as good as those of the semiconductor detector.